The scanning tunneling microscope, hereinafter STM, is an instrument capable of resolving surface detail down to the atomic level. The microscope's conductive tip, ideally terminating in a single atom, traces the contours of a surface with atomic resolution. The tip is maneuvered to within a nanometer or so of the surface of a conducting substrate so that the electron clouds of the atom at the probe tip overlap that of the nearest atom of the sample. When a small voltage is applied, electrons tunnel across the gap between the microscope tip and the substrate, generating a tunneling current the magnitude of which is sensitive to the size of the gap. Typically the tunneling current decreases by a factor of 10 each time the gap is widened by 0.1 nanometer.
Movement of the microscope tip is controlled by piezoelectric controls. In one mode of operation, the tip or probe is held at a constant height as it is moved horizontally back and forth across the sample surface in a raster pattern, its parallel tracks separated by a fraction of a nanometer. This causes the tunneling current to fluctuate and the current variation is measured and translated into an image of the surface. The current increases when the tip is closer to the surface, as when passing over bumps such as a surface atom, and decreases when the tip is farther from the surface, as when passing over gaps between atoms. In an alternative mode of operation, the probe or tip moves up and down in concert with the surface topography as it is moved across the surface in a raster pattern. Its height is controlled to maintain a constant tunneling current between the tip and the surface. The variations in voltage required to maintain this constant gap are electronically translated into an image of surface relief.
The image obtained by either mode of operation is not a topographical map of the surface, but a surface of constant tunneling probability affected by the variations in the occurrence and energy levels of the electrons present in the surface atoms. If the surface is composed of a single type of atom, the image may closely resemble topography, but when various atoms are present pits or bumps will appear in the image depending upon their electronic structures.
Further detail on the structure and operation of the STM is disclosed in U.S. Pat. No. 4,343,993 of Binnig et al issued Aug. 10, 1982; Wickramasinghe, H. K., Scientific American, October (1989) pp. 98-105; Rabe, J. P., Angew. Chem. Int. Ed. Engl. 28 (1989) No. 1, pp. 117-122; and Hansma et al., Science, 242, Oct. 14, 1988, pp. 209-216.
The STM is useful not only for the imaging or characterization of surfaces, but also for manipulating surfaces on a scale as small as subnanometers. STM is limited to imaging or manipulating surfaces which conduct electrons. Therefore, thin conductive coatings or replicas have been used on substrate surfaces which are nonconducting. See McCord et al., J. Vac. Sci. Technol. B 4 (1), 86-88 (1986); McCord et al., J. Vac. Sci. Technol. B 6(1), 293-296 (1988); Research Disclosure 28130, September 1987; Schneir et al., J. Appl. Phys., 63, 717-721 (1988); and Schneir et al., Langmuir, 3, 1025-1027 (1987).
Metal deposition onto a substrate surface from a gas is another method which has been used to pattern lines using the STM. See McCord et al., J. Vac. Sci. Technol. B 6, 1877-1880 (1988); Japan Patent Application 63-271,743 published Nov. 9, 1988; and U.S. Pat. No. 4,550,257 of Binnig et al., issued Oct. 29, 1985. Metal deposition onto gold is taught by Emch et al., J. Appl. Phys., 65, 79-84 (1989). Deposition of particles onto the surface from a carrier is disclosed in U.S. Pat. No. 4,829,507 of Kazan et al. issued May 9, 1989.
The formation of protrusions or raised surface areas on metallic glasses by local heating using the STM has also been reported as a means of nanometer lithography by Staufer et al., J. Vac. Sci. Technol., A 6, 537-539 (1988) and Ringger et al., Appl. Phys. Lett. 46, 832-834 (1985).
Writing using the STM wherein the microscope tip physically touches, scratches, indents, or creates holes in the substrate surface has been taught by Van Loenen et al., Appl. Phys. Lett., 55, (13), 1312-1413, Sep. 25, 1989; Jaklevic et al., Physical Review Letters, 60, 120-123 (1988); and Garfunkel et al., Science, 246, 99-100, Oct. 6, 1989.
Another approach to writing with the STM has been to use the tunneling current for surface rearrangement of the atoms already present. Such lithography has not been reliably reproducible. See Becker et al., Nature, 325, 419-421 (1987), and Japan Patent Application 63-60,196 published Mar. 16, 1988.
The ability to manipulate single atoms or molecules with the STM provides unique potential applications in microelectronics. Much of the current interest in high temperature superconductors is related to their use in electronic applications. One important criterion in this area is the ability to make extremely fine structures such as junction devices. Also of great importance is the surface chemistry and surface morphology of these materials.
R. Laiho et al., Journal of Microscopy, Vol. 152, Part 2, pp. 407-413, November (1988) disclose investigation of the surface structures of the high temperature superconductors YBa.sub.2 Cu.sub.3 O.sub.7-x, Bi-Ca.sub.1.7 Sr.sub.0.7 Cu.sub.2 O.sub.x and TlCaBaCuO.sub.4.5.+-.x using STM. H. Heinzelmann et al., Journal of Microscopy, Vol. 152, Part 2, pp. 399-405, November (1988) discuss use of STM and of AFM (atomic force microscopy) at room temperature and pressure to investigate the surface of single crystal YBa.sub.2 Cu.sub.3 O.sub.7-x high temperature superconductors. Both of these references teach imaging or characterization of surfaces but no means of etching or lithography on these superconducting substrates.
H. Heinzelmann et al., App. Phys. Lett., Vol. 53, No. 24, p. 2447 (1988) disclose investigation of the topography of the surface of the high temperature superconductor HoBa.sub.2 Cu.sub.3 O.sub.7-x single crystal using STM. Surface features have been modified on this material using STM at a high threshold voltage (.gtoreq.2.0 V). This work was done only on single crystals. Etching by multiples of layers (a few angstroms) was not demonstrated and the resulting etch pattern was irregularly shaped indicating an uncontrolled process. Thin films may be of more practical use for devices.
It is therefore an object of the present invention to provide a process for etching of nanoscale structures using STM on a high temperature single crystal or thin film superconductor.
It is a further object of the present invention to provide a lithography process using a STM on a single crystal or thin film superconductor which requires no post writing treatment to stabilize the image.
It is a further object of the present invention to provide a lithography process using a STM on a single crystal or thin film superconductor which can controllably remove one or more layers in the a-b plane and which does not destructively deform the superconductor.